Sugar Manufacture
Outline of the modern manufacturing process 


 

Sugar factory site

Cane Milling

a. Cane preparation :  The first process in sugar manufacture in a sugar factory is the preparation of cane, before it is sent for crushing.  The cane stalks are carried in the cane carrier by two endless chains of roller type and placed at an angle.  They are cut into chips by set of revolving knives and are also torn into shreds by hammer mill shredders to hasten crushing and extraction.
 


 

Cane feeding into carrier

Motors in sugar industry

b. Cane Crushing : A sugar mill consists of multiple units 4 to 6 of three roller combinations , 12 to 18 rollers in all.  In each set, three rollers are arranged in a triangular form and are called:
i. Cane of feed roller
ii. Top roller and
iii.Bagasse or discharge roller.

            Steam engines or turbines are used for driving the mills.  Hydraulic pressure on the top roller enables extraction of maximum juice from the crushed cane leaving bagasse.  This final bagasse or crushed cane is wood fibre containing about 48% to 50% moisture and is generally used in the boilers as fuel but can be used for manufacture of paperboard or paper. 

 


 

Mill house

Self Setting mills

c. Imbibition :

            Sprays of water or thin juice are directed on the blanket of bagasse as it emerges from each mill and this process is called imbibition or maceration.

d. Sucrose extraction :

            Adoption of modern methods in imbibition enables extraction of over 93% of sugar in cane and this is known as sugar extraction or simply extraction.  The mixed juice contains particles of the bagasse which are removed before the juice goes for clarification.

Diffusion 

            In some sugar factories, Desmedt Diffuser is used and also Dds and Saturans.  Here cane is prepared by two sets of cane knives and one set of mills for primary extraction of about 65% to 70% of the sugar content and the diffuser extracts the remaining sugar in bagasse.  This bagasse contains 85% moisture, which has to be reduced to about 48% - 50% before it can be used as fuel in the boilers. As the slurry of prepared cane material is passed through the diffuser with conveyor over the different hoppers of the diffuser, it is subjected to intensive sprays of juice of progressively decreasing concentration.

            The bagasse coming out of the diffuser contains large quantity of residual juice which is squeezed out by passing residual juice extracted by the dewatering mills is screened and milk of lime added to bring the pH of the juice to 9.0 heated to 80C and sent to the clarifier for settling the suspended impurities.  Operation of the diffuser is fully automatic with minimum manual supervision called purging or curing.

Processing

      a. Clarification :

            The juice from the mills is turbined, dark green in colour, acidic in character and contains both solubles and insoluble impurities.  These are by using chemicals and heat as clarifying agents and this process is called clarification.  Heating and filtering removes the colloids, which cause turbidity in juice.  Lime treatment removes gums, pectins, organic and inorganic acid salts by precipitation.  Colouring matter is removed by treatment with sulphurdioxide.  The juice extracted by the mills is passed through a metal strainer to remove suspended purities.  Use of clarifying agents like milk of lime and sulphurdioxide helps in precipitating non-sugars as mud whose removal is essential to prevent interference later on in separation of sugar crystals from the mother liquor.  The mud gets separated from clear juice by sedimentation in clarifiers and or filtered with the help of rotary drum vacuum filters.
 

The precipitate from the juice is called press cake, and is used as fertilizer and in preparation of other by-products while the clarified light juice is sent to the evaporators. Two processes are vogue in clarification the more common one is sulphitation and the other is carbonation.   

b. Sulphitation :

            Milk of lime is added to the raw juice to neutralize the acidity of juice and to keep the juice alkaline upto ph 9.0 - 10.5.  Passing sulphurdioxide gas neutralizes excess lime.  Sulphurdioxide is used as the chief bleaching agent to make the juice brilliant and light in colour.  The juice treated with lime and sulphurdioxide is heated to boiling point.

c. Carbonation :

            In this process, passing carbon dioxide gas resulting in precipitate of calcium carbonate neutralizes excess of lime.  The lime stone mixed with coke is burnt in a special kiln to obtain burnt lime as well as carbondioxide, carabonization is a costlier process than sulphitation as the requirement of lime is high.  The whole juice after treatment is filtered using plate frame type filters.  But the latest system is to have continuous clarification in a single tray-type clarifier with vacuum filters.

d. Evaporation under vacuum:

            The clarified juice contains about 85% water, two thirds of which is evaporated in a series of vacuum boiling cells of multiple. Effect evaporators thus avoiding destruction of sugar caused by boiling of juice over direct fire or by steam in all.  This also results in better fuel economy.  These evaporators are arranged in a series and the juice is boiled in such a way that each succeeding evaporator has a higher vacuum and therefore boils at a lower temperature.  The juice thus becomes more and more concentrated and in the last evaporator turns sugar into syrup containing about 60 - 65% solids and the rest water.

e. Sugar boiling and crystallization:

            The evaporated syrup is black in color.  Sulphurdioxide is passed through this syrup to bleach the colouring matter.  The sulphured syrup is then evaporated till it gets saturated with sugar crystals and as the water evaporates syrup is further added.  The concentration of syrup and formation of crystals is achieved in vacuum pans. 

Pan boiling is an important and skilled operation as the size and colour of crystals of the final product sugar are determined by the quality of pan boiling.  When formation of new crystals ceases, final concentration is done and the massecuite called strike is then discharged into crystallisers.

f. Centrifuging:

            The massecuite from the crystallisers is pumped into a revolving machine called centrifuge.  Washing with water and then stemming under high speed, the centrifugal force of the centrifuges is increased expelling molasses into the casing of the machine, retaining sugar crystals on the perforated sheet.  Spinning is continued till the sugar crystals are practically free from molasses.  Such separation is called purging or curing.

            In modern times, centrifugal machines are of a continuous type working at a speed of 2300RPM resulting in lower operation cost and higher efficiency.  The use of superheated sugar but on the other hand the sugar has to be cooled in a cooler before it is packed in a Twill jute bag.

            The final molasses, which is heavy and viscous, contains about one third sucrose, one-third reducing sugars and the rest non-sugars.  It serves as raw material for production of industrial alcohol and yeast and is also useful as cattle feed. 

 


 

Centrifugals

 
Grading, Packing & Storing

a. Grading

            Grading of sugar is done in sugar grades of vibrating type with different decks fitted with various meshed and the sugar from each deck is collected through separate chutes into the bags directly.  In India, besides size of grain, sugar is classified according to colour also.  Presently there are only two colours 20 and 30 the latter being superior to the former.  Recently under new gradation, Government of India reduced the existing five-grain sizes to three grades, viz., large, medium and small, the two colour series remaining as they are.   The gradation has come into force from 1984 - 85 crushing session.
 


 

Sugar Crystals

 

b. Packing and weighing:

            Sugar is packed in 100 Kg. bags, which are stitched mostly by machines.  Sugar for export is packed in 50Kg. special gunny bags having polythene lining to withstand multiple handling and transportation to long distances.

c. Storing

            Sugar bags are stored in pucca godowns mostly by manual labour.  In many of the factories stacking is now being done by conveyor belts and stacking equipment. The major portion of worlds sugar production is raw sugar, which is mostly produced from sugarcane and also from sugar beet. The setting up of sugar factories for producing raw sugar in many countries were importing raw sugar from colonies and refining them in their own countries throughout the year. 

It is manufactured from cane juice or from beet in the usual manner by defecation except that the massecuite during centrifuging is not completely washed with the result that the film of molasses surrounding the crystals is allowed to remain as such, imparting brown colour to the sugar with a purity of 97% to 98%. Producing raw sugar helps in increasing crushing capacity by about 25%.  Also the molasses produced becomes edible because sulphur is not used in the processing.  A serious disadvantage however is that raw sugar deteriorates very fast during storage.

            Refining of raw sugar is done by softening the film of molasses and centrifuging the mixture of sugar crystals and syrup.  It is then boiled in vacuum pans and crystallizes out as refined sugar. During crystallizing and centrifuging, all colouring matter and ash are removed resulting in production of refined sugar of 99.95% purity as against 99.5% of white sugar produced in India, which may in few cases go upto 99.8%.

Cane By-Products 

            The Sugarcane plant is noted primarily as the raw material for manufacture of sugar but it is essentially a multi-purpose plant from which a number of industrial and other products can be derived.  The cane tops that are removed before sending cane for crushing form 25% to 30% of harvested cane and serve as good fodder for cattle.  The main by products in the manufacture of sugar are bagasse, molasses and filter cake besides furnace ash - all of which form about 40% of the weight of cane crushed.  Each of these by-products serves as source material for manufacture of several industrial products, which are in great demand.

            The development of sugarcane by-product industries and their ancillaries improves the profitability of the sugar industry and reduces the production costs of sugar.  Several sugarcane producing countries like Australia, Brazil, Cuba, Philippines, South Africa and Taiwan have developed numerous industries utilising the by-products of the sugar industry as source material.  India is comparatively lagging far behind in this aspect and unless these are developed rapidly Indian sugar will not be able to compete in the World sugar market. 

Bagasse :

            Bagasse forms about 30-35% on weight of cane harvested and is the chief by product in the sugar industry for use as a renewable source of energy.  In fact power from bagasse in addition to the potential use of alcohol from molasses as a source of fuel indicates cane sugar industry as a most remunerative industry in the country.  The greatest advantage being its price and energy potential. 

            Bagasse contains 20-27% pith by weight and 5% non-fibrous epidermis and the rest is fibre.  If pith is separated from bagasse.  The depithed bagasse can be processed into bleached pulp or semi pulp by chemical process and can form an excellent raw material for production of all types of high grade paper.  Presence of pith in bagasse is detrimental to production of quality paper but on the other hand when it is separated, pith can be used entirely as fuel in boilers. 

                        Recently it has been reported from Canada that surface of cane stalk can be separated into rind, pith and epidermis and this separation helps in processing new wood fibre for pulp and paper for use in panel boards.  Pith can be utilised also for production of furfural, which is used in industries, besides its utility in boilers. Bagasse ash can be used in the manufacture of chemical resistant glass.
 


 

Paper Mills with Bagasse as raw material

Power supply from sugar factories

 

Molasses :

            Molasses forms 3 to 5% of weight of cane crushed.  It is the mother liquor in final stage of manufacture of sugar in a sugar factory out of which further crystallisation or separation of syrup is not possible.  The products that can be manufactured from molasses include alcohol used for industrial purposes, citric acid, oxalic acid and lactic acid for industrial application and carbon-dioxide  used in production of dry ice.  Sugars present in molasses can be utilised for production of cattle feed, edible syrup, solvents and chemicals like ethanol (rectified spirit), absolute alcohol, etc., which can be mixed with petrol in certain proportion which is then called power alcohol.

Molasses usually contains potash (7.5% K20),  P2O5 (0.5%) AND Nitrogen (0.2%) and can be used after proper dilution with water as a fertiliser particularly for reclaiming alkaline soils.  The manufacture of industrial alcohol from molasses is presently of significant importance but with the availability of naphtha from oil refineries, use of molasses for this purpose may get diminished but a point in favour of molasses is that it is a renewable source unlike naphtha.
 


 

Storage Tank of molasses

Distillery Effluent :

            Distillery Effluent or spent wash or spillage is the liquid left over after the alcohol is removed during fermentation and distillation.  It has been reported that the effluent can be used for commercial production of methane, which serves as fuel.  Distillery effluent can also be used as manure when mixed with irrigation water.

Filter Cake or Press Mud :

            Filter Cake or Press Mud forms 2.5 to 3.5% of cane crushed in sulphitation factories and 7% in carbonation factories.  Sulphitation filter cake contains 20-25% organic matter and about 45% lime on dry basis.  It is useful as a manure because it contains small quantities of nitrogen, phosphate and potassium and is utilised in reclamation of alkaline soils and serves as a basal dose replacing farm yard manure in general cultivation of sugarcane.

            It has been recently reported from Cuba that fresh and stored press mud can produce bio-gas and can constitute a renewable source of energy of considerable scope. Carbonaceous filter cake is rich in lime but is not of practical importance.

Cane Wax :

            Cane wax, that is originally present on the surface of sugarcane stalk as a whitish deposit can be extracted from sulphited filter cake which contains about 3% wax Crude cane wax can be used directly in the manufacture of carbon paper, in polish industry as protective coating on fruits and for preparation of waxed papers. Wax recovery is about 10% on dry filter cake and it is advantageous to extract wax first before filter cake is used as manure.  When all the byproducts produced in a sugar factory are converted into useful industrial products as indicated above the financial returns from the byproducts would far exceed those from sugar itself.
 

Utilization of Sugar Industry by-products

            The cost of production of sugar continues to be high, due to the neglect of the profitable utilization of the by-products.  If the by-products are used the cost of production of sugar may go down by about 20%. The chief by-products of the sugar industry broadly may be taken as (a) bagasse, (b) molasses and (c) filter mud.

Molasses  : 

                        Molasses is an important by-product of the sugar industry constituting about 4.2 to 4.5% on cane utilised for sugar production.  Molasses contains about 28 to 30 % sucrose, 16 to 20% reducing sugars and a substantial quantity of aconitic acid, besides valuable nitrogenous substances, potassium salts and phosphates. 

            It is the most industrially important raw material for manufacture of a variety of products such as distillery products, chemicals, lactic acid, citric acid, baker's yeast, acetone, and chemicals in various industries. Molasses will be a very good source for industries requiring acontic acid in the near future Our efforts should be directed to develop a suitable method of recovering the particular acid. Presently, the conversion of molasses by the process of fermentation and distillation is extensively used in the manufacture of methyl and ethyl alcohol and potable spirit.  It can conveniently be converted into absolute alcohol which can be used as an admixture to power alcohol.  Molasses is also utilised in the manufacture of animal feed.
 

Bagasse :

            This is a by-product of the sugar industry and not a waste product presently used as fuel in sugar factories, where power and steam are important cost items in manufacturing. Bagasse is rich in cellulosic materials and can well be utilised for pulp and paper,  paper-board, corrugated board, insulating hard-board products for the building industry and for providing insulation against heat, cold and sound.   Bagasse on dry weight contains about 18% lignin, 15% water soluble substances, 45% cellulose, 28% pentosans and other hemi-cellulose. Because of its long, tough, springy character, bagasse is a superior raw material for the manufacture of insulating building board products and that it is not possible by known means to produce a fibre of similar properties from wood, bamboo, hesian etc.  Intensive experiments conducted in America have shown that fibres of the rind and bundles are excellent for producing writing and fine papers.

Chemicals :

            Bagasse is a source of not only useful fiber but also an excellent source of chemicals like formaldehyde, furfuraldehyde and also plastic materials.  As bagasse contains a large proportion of pentosans, it is a very suitable raw material for the production of furfural which has a wider range of uses, from the synthetic manufacture of rubber and nylon to the separation of vegetable oils into food and oil paint constituents.  Furfural is manufactured either by batch or continuous process of digesting it with sulphuric acid at definite temperature and pressure and separating it from other components.  Alpha cellulose is highly purified cellulose used mainly in rayon, acetate fibers, cellophane plastics, explosives, photographic films, lacquers and fine papers.  Needless to say lack of technology is no longer a bottleneck and prospects for utilization of bagasse have brightened.